Common-frequency broadcasting system



March Z5, 1930. I Ejl. GREEN l .1,751,516

I COMMON FREQUENCY BROADCASTING SYSTEM V Fiied Aug. 8, 1927 2sheets-sheet 1 I?. 4- Sil/Lyle )cirque/ug @rang/men @,W@ V@D @y @C -kreeReyner/wg vwzgemfn .2,1

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E.v i. GREEN COMMON FREQUENCY BROADCASTING SYSTEM' Filed Aug. 8, 1927Filter n F12/tel Y ffy' March' 25, i930.

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JNVENToR. E L @www ATTORNEY Patented Mar. 25,193() l UNITED STATES#PATENT orifice i v .'ESTILL I. yGr]EtEEl\T,0F EAST ORANGE,v NEW JERSEY,ASSIGNOR'TO AMERICAN TELE- PHONE AND TELEGRAPH COMPANY, A CORPORATION OFNEW YORKl COMMON-FREQUENCY BROADCASTING SYSTEM l Application filedAugust Y8, 17927. `Serial No. 211,518.

This invention relates to systems for broadcasting programs' by means ofradio stations, and, more particularly, to 'systems in .which aplurality Vof radio stations are arranged to broadcast simultaneously atthe same wave length.

order to conserve the available fre' quency range, it has heretoforebeen proposed that a number of broadcasting stations be arranged tooperate on a common fre#A broadcasting at a common wave length, therewill be a certain number oflisteners who' are so located with respect tothe stations asVV to receive similar field strength fromtwo stations, sothat interference"results,y c

In general, two Vdifferent cases -of Vinterference in a common frequencybroadcasting system may'arise, namely, interference where twoV stationsytransmit differentgprograms but on a common wave length, andinterference arising where the stations not only broadcast at a commonwave length, but broadcastV the same program. In either of these cases,

a listener located in a zone where the field strengthV of the twonearest stations has the same order of magnitude will experiencesubstantial interference between the signals from. the two stations. Fornormal trans mission conditions and equal power atthe utwo broadcastingstations, this interference zone would obviously lie approximately mid#waybetween stations. c,

This interference may take several differ# ent forms.` `If the carrierfrequencies of the two stations are not accurately synchronized, audiblebeat notes may beV produced between the two carrier frequencies.,vFurthermore, even though the carrier frequencies be kept suiiicientlyclose together to avoid audible beats, there will be, in the case ofstations broadcasting different programs Vatfthesame wave length,considerable diificulty,duer to cross-talk between the two different'pro# 4In the case of `stations broadcasting the same program, there willa pulsating or fluctuating effectif the difference between the twocarrier frequencies is a fewcycles,

or slow fading if the carrier frequencies are more accuratelysynchronized. v Y In `accordance with the present invention, 1t isproposed to overcome these diiculties jby employing several broadcastingfrequencies, each common to a number of stations,

the stations being so physically located with l reference to eachothervthat where a receiver 1s located in an interference zone betweentwo stations transmitting on the same lwave length, reception willbe'possible from another broadcasting station employing a diflferentwavelength, without substantial interference from another station alsoemploying such different wave lengths. While the 'invention isapplicable to a chaincof broadcasting stations employing the common wavelengths, whether they transmit the same or different programs,:it ismore particularly applicable to the situationin which the same programis transmitted. Y

The invention may now be more fully 'un- .derstood from the'v followingdetailedde` scription when read in connection with'the accompanyingdrawing, vFigure 1v of which shows symbolically a lay-out ofbroadcasting stations all transmittingl at the same lwaveV yfrequenciesin such a mannei as to entirely eliminate allinterfe'rence zonesbetweenstations; and Fig. 6 of which'represents schematically al circuitarrangement whereby a plurality of broadcasting stations maybeinterconnected in accordance with the present a plurality of wavelengths,each wave length being employed incommon by several 'stai tions.Y l

Referring to Fig. l, small circles represent broadcastingstationsarranged generally in rows; The numeral l appliedto each circle.invention to broadcast'the sameprogram on .ery underlying thisindicates that each broadcasting station so designated employs the samecarrier frequency. In such an arrangement, if we consider a receiverlocated between the first and Second stations at the left of the upperrow, it will be apparent that interference will occur. Likewise,considering the first station at the left in the upper row, and thesecond station from the left in the lower row, there will beinterference in a receiver located substantially midway between the twostations. In general, interference zones may be eX- pected to occur overthe cross-hatched areas of Fig. l.

By using two different frequencies and assiging one or the other ofthese frequencies tothe stations, as indicated in Fig. 2, theinterference Zones will be reduced to very small areas. In this figure,all of the stat-ions marked l are transmitting at one common wavelength, and all of the stations marked 2 are transmitting at anothercommon wave length. It is to be noted that alternate stations employdiiferent frequencies. The thearrangement is that a receiver located ina Zone where interference might be expected between two stations usingthe same frequency may be tuned to the other frequency provided by thearrangement, in which case the receiver will not in general receiveequal signal intensities from the two nearest stations employing` thealternate frequency. From a consideration of the arrangement of Fig. 2,however, it is apparent 'that although thesituation has been greatlyimproved, there still remain certain small regions in which the listenerwould receive two signals of approximately the same intensity from thetwo nearest stations, regardless of'whether he tunes his receiver to theone wave length or the other. For example, a receiver located midwaybetween the first two stations at the left in the upper row may receivethe program by tuning either to the first 0r to the second wave length,without interference, although the receiver is substantiallyequi-distant between the two nearest stations. The same would be truefor a re ceiver located between the extreme left-hand stations of theupper and lower rows, or for a receiver located between the stationssecond from the left in each of the rows, etc. In the case, however, ofa receiver located about midway between agroup of four adjacentstations, such, for example, as the group comprising the first andsecond stations from the left in the two rows, interference would occur, whether it was tuned to the first wave length, or the second. i'Such a receiver would be substantially equi-distant from the two neareststations employing the first wave length, and also substantiallyequi-distant between the two nearest adjacent stationsemploying thesecond wave lengt-h.

By employing three different frequencies for the system, it will bepossible, by properly locating the stations, and properly assigning thefrequencies to the stations, to entirely avoid the presence of any Zonesof in terference. In fact, theoretical considerations indicate thatthree frequencies Constitute the minimum number with which such acondition is possible. One such an arrangement is illustrated in Fig. 3.Here, the broadcasting stations are arranged substantially in rows, withstations in any row staggered wita respect to the stations in anadjacent row. The numerals l, 2 and 3, applied to the stations, indicatethat those stations designated by the numeral l broadcast at one commonwave length, those designated by the numeral 2 at a second common wavelength, and those designated by the numeral 3 at a third common wavelength. If we consider any group of four nearest adjacent stations,surrounding a common area, it will be seen that while two of them employthe same wave length, the other two will employ different wave lengths.Therefore, any listener in the interference Zone between the twostations employing the same wave length may tune his receiver to one orthe other of the two remaining stations without substantialinterference. For example, take the first and second stations from theleft in the upper two rows. A listener midway between the first stationfrom the left in the second row, and the second station from the left inthe top row would be in an interference zone with reference to these twostations which broadcast at the same frequency. By timing to thefrequency of the first station at the left in the top row, or by tuningto the second station from the left in the second row from the top, sucha listener should be able to receive a program from either of thesestations without substantial interference. S0, also, if we consider alistener located at the point in Fig. 3, it will be apparent that suchlistener will be midway between two adjacent stations, both broadcastingat frequency No. l. If he tunes to frequency No. 3, which is employed bythe second station from the left in the second row from the top, thenearest station transmitting at this same wave length will be the firststation at the left in the lower row. It may be shown geometrically thatthe distance from the listener to the last-mentioned station isapproximately 2.65 times as great as the distance from the listener tothe second station from the left in the second row from the top. Thelistener would therefore be quite favorably located with referencev tothe nearest station employing frequency No. 3, and would not besufficiently close to any other station employing the same wave lengthto meet with any considerable degree of interference.

An alternative arrangement, employing three frequencies, is illustratedin Fig. 4.

n station from the yleft in the lower row, which',

Here, the. stations. are located substantially" in rows, bothvertically` and horizontally. Two frequencies are employed for thestations of each horizontal row, so that succes.-

, sive stations broadcast at alternate frequencies.'V Successivestations 1n each vertlcal row,

however, employ dierent ones of three fre'- quencies. group of fournearest stations surrounding. a common area, 'it will be seen that onlytwo of the four stations. employ. the same Wave:

length. Any' listener, inrthis kcommon area, who is substantiallyequi-distantbetween the two stations `employing the 'same wave lengthvmay, however, tune to either of the other' stations without materialinterference. Any listener in the common area, who is not substantiallycentrally locatedwith respectto the four. stations, may tunehis'receiver to any Vone of the three frequencies employed in thevsystem, without substantial interference.

Consider, for example, alistener located at Such a listener, by tuninghis receiver to'frequency No.` l, would encounter interference betweenthethird station fromthe left in the toprow, :and the. second stationfrom the left in the second row. If, however, he yshould tune hisreceiver to frequency No. V3, the. nearest station broadcasting at that.frequency will be the third station from the left .in the second row.yThe next nearest station broadcasting at this frequency will be thesecond station is geometrically located approximately 2.24 times as farlfrom the station m as is the third station from the-left in the secondrow..

Therefore the listener atk w should not en counter material interferenceat .the third frequency. In a similar manner, Ahe. can. tuneto frequencyNo. 2 without encountering` any greater degree of lnterference than .inthe..

case of frequency No. 3.

Still another arrangement, employing.

three frequencies, .is illustrated .in Fig. `V5.

Here, as in Fig. 3, thejtransmitting stations,

are arranged approximately. in rows, with` the stations in alternaterows staggered with respect to those in the adjacent rows. Thelarrangement ofFig. 5 differs from-,that of Fig. 3 principally in thefact that .thetrivangle formed by-any three Vadjacent lstationsissubstantially a right angle y'triangle .rather than substantially an.equilateral triangle. Here, again, if we consider, any group offfoujradjacent stations surrounding a common p area, it will be vseen that notmore thantwo of them transmit at ythe same-carrier frequency.

If we consider a listener located at m, suchy a.

. listener will be .substantially l:equi-distant,

with respect to the second `station from 4the left in the upper rowandthe lower row, and.

will therefore meetf'withV interference from those stationsreach .ofwhich will .betran's-` mitting ,frequency'Nd 2.. .Such a listener; may`tune his .set to: either frequency No. 3

Here, again, if we considerany or frequencyNo.l 1.1.If he tunes to"frequeny cy No. 3 for example, itwill be seen that the nearest stationtransmittingzL-at that frequeD-f cy is the firststation at the left inthe middle row. The nexty nearest station is the third station from theleft in the upper row, andk it may be shown geometrically that thelatter stationis approximately 2.24 times asfar` from station a as isthe first station at the left inthe middle row.y It will be understoodthat the arrange'- ments above described represent ideal loca; A

tionsoffagroup of broadcasting stations.y In'` practice, it will benecessary, due to local con ditions,l tov depart, to some extent, fromany one of the above schemes which vmight bekk chosen. Nevertheless, itshould be possible,

in-any practical. case, to arrive'at a suiicientv. approximation of oneof the above idealar:-v

rangements. to insure that a listenerlocated at any point will be ableto receive a signal from at least one station yof the four neareststations without undue interference. It will also be understood thatvotherr arrangements y of stations and frequency allocationsfollow,v

linked together as ,to broadcast th-e same pro-` gram, utilizing `threedifferent .carrierL vfre-.r

quencies for the `'entire system. The voicev fre,- quency programoriginates at and aI syn.- chronizingfrequencyis generated at'S. The

synchronizing frequency is' impressed lupon a frequencystep-up.arrangement .HG1 .at .the

local radio .stationfA to `generate'a carrier frequency f1. The,frequency step-upf Larrange-- ment may be, for example, a harmonicgenerator, of'known type, -upon which the synchronizing ,frequency maybe impressed, to.- gether with a selective device to select the de.-sired harmonic of the 'synchronizing frequency.r f The Vde isiredharmonic is` then impressed upon the radio r`transmitter Ta. The

frequency Vstep-upy device may .also includey suitable amplifying means,if necessary. Y The prog-ram from vV is also impressed upon the radiotransmitter Ta to modulate the selected carrier frequency. v

The program originating lat V, and .the synchronizing frequencyvoriginating .at 1S, may.

alsov be applied to a .transmission line ,Li through filters VF and SF.The line L1- may include .one or *more repeaters, such as l, and serves`to transmit the program :and-synchro? ii'izing Yfrequencies to stationB.- Here, ithe program and synchronizing frequencies areselected throughfilters VFb-,alldl SF, .respec-` tively,'and impressed upon a radiotransmitj station B, which may be assumed to be a frequency f2.' v

` 'I he program and synchronizing frequencies are transmitted from' thestation. B over a line L2 (which may include a repeater or repeaters R2)to a station C, which is provided with equipment similar to that atstation B, except that here the frequency step-up device HGrc steps upthe synchronizing frequency to a carrier frequency f3.

Similarly, the program and synchronizing frequencies may be transmittedover other line circuits such as L3, L4, etc., toV other stations, suchas D, E, etc., where additional carriers f1, f2, etc., may be generated.

,In order lthat the program maybe impressed upon each radiotransmitterat exactly the same time, regardless of the length of Ylineover vwhich the program is transmitted,

delayfcircuits, such as Da, Db', etc., are interposed in the programconnection leading to each radio transmitter, these delay circuits beingso 'designed that the over-all transmission time from the program sourceto each radio station will be the saine.y Phase correcting devices, suchas Pa, Pb, etc., are also provided at each transmitting station in theconnection leading to the frequency step-up device for the purpose ofVadjusting the phase of the arriving synchronizing frequency, so

' that it arrives at each station in the `same phase relation. Thedelay/'circuits and phase correctors may be of any known type. For

Y example, a delay circuit may be a network of the Campbell filler type,and the phase corrector may be anything "from a simple, adjustablecondenser to a more complicated adjustable networlrfor phasecorrection.V

It will be obvious that the general principles herein disclosed may beembodied in many other organizations widely different from thoseillustrated, without departing from the spirit of the invention asdefined in the following claims.

What is claimed is:

1. In a chain broadcasting system, aplurality of broadcasting stationsarranged to transmit the same program, each station transmitting on buta single wave length,

, said stations being arranged lin groups, each station of a grouptransmitting at the same wave length, but more than one transmittingWave length being employed for different groups and all of the groupssimultaneously transmitting the same program, the stations of thedifferent groups being so located with respect to each other vand thestations of other groups,.a'nd t-he Wave lengths being so assigned tothe stations, that any set of four closely adjacent transmittingstations surroundingA a common area will employ less thanfour but notless than two carrier frequencies.

- 2. In a chain broadcasting system, a plurality of broadcastingstations arranged to transmit the saine program, each stationtransmitting on but a single wave length, said stations being arrangedin groups, each station of a group transmitting at the same wavelength,'butimore than one transmitting Wave length being employed for differentgroupsand all of the groups simultaneously transmitting the sameprogram, the stations of the dierent groups being so located withrespect to each other and the stations of other groups, and the wavelengths being so assigned to the stations, that any set of four closelyadjacent transmitting stations surrounding a common area will em-` ployless than four but not less than three wave lengths.

3. In a chain broadcasting system, a plurality of broadcasting stationsarranged to transmit the same program, each station transmitting on `buta single Wave length, said stations being arranged in groups, eachstation of a group transmitting at the same wave length, but more thanone transmitting wave length being employed for different groups and allof the 'groups simultaneously transmitting the same program, thestations of the different groups being so located with respect to eachother and the stations of other groups, and the wavelengths being `soassigned to the stations, that of any set of four closely adjacenttransmitting stations surrounding a common area, certain stations of theset will employ the same Wave length but not more than two will employthe same wave length.

4. In a chain broadcastingV system, a plurality of broadcasting stationsarranged to transmit the same program, eachV station transmitting on buta single wave length, said stations beingarranged in groups, eachstation of a group, transmitting at the same wavelength, but more thanone transmitting wave length being employed for different groups and allof the groups simultaneously transmitting the same program, the stationsof the different groups being so located with respect to each other andthe stations of other groups, and the wave lengths being so assigned tothe stations, that of any set of four closely adjacent transmittingstations surrounding a common area, certain stations of the set willemploy the same wave length but not more than two will employ the samewave length, and the other two stations of the set will employ differentwave lengths.

5. In a chain broadcasting system, a plurality of broadcasting stationseach transmitting on a single wave length and arranged substantially inrows, and a plurality of transmitting Wave lengths simultaneously'carrying the same program being so assigned to the Stil stations thatcertain stations of all of the rows employ the same transmitting wavelength7 and certain other stations of all the rows employ differenttransmitting wave lengths, the wave lengths being also so assigned tothe lstations that any station in a given row will transmit on afrequency different from the transmitting frequency of the nearestadjacent station in the same row, and from the transmitting frequency ofthe nearest adjacent station in the nearest adj acent parallel row.

In testimony whereof, I have signed my name to this specification this2nd day of August, 1927. o

ESTILL `I. GREEN.

